Concurrent with the advanced visual and color features that have found applications in printed materials during recent years, there has been an increased demand for printing papers having higher quality. On the other hand, there is a great demand for weight reduction in printed materials for the sake of reduced costs in transportation and mailing. Traditionally these two demands have been mutually contradictory, given that high-quality coated printing papers are conventionally characterized by higher basis weight of the base paper and greater coating weight, as well as higher density for a given basis weight due to smoothing through surface treatment. A paper with a lower basis weight may be selected in order to reduce the weight of a printed material. However, that is not an ideal solution, since using such a means of weight reduction without changing the density will result in thinner paper and diminish the feeling of bulk expected of a book. For the above reasons, the market is presently demanding high-quality coated papers that ensure higher bulk; in other words, which offer greater paper thickness at a given basis weight or a lower basis weight at a given paper thickness, and which meet the criteria required of coated papers used for upscale printing applications.
Recently there has also been a trend of public preference for small-size, handy information magazines such as the so-called “mook” (magazine-format book) and “pocket guide.” Pliability is one of the important features required of papers used for these publications. If a rigid paper is used for such magazines, the smaller the size of the book becomes, the more easily the pages will stand straight as they're flipped up and over, making it extremely inconvenient to open and read the book while holding it with one hand, for example, when one is on the road. One of the indicators used to measure the level of paper pliability is the Clark stiffness tester. Paper stiffness increases in proportion to the cube of the paper thickness. If the paper thickness is increased to gain higher bulk at a given basis weight, the paper stiffness increases accordingly. Given the above, it has traditionally been considered extremely difficult to achieve a paper offering excellent pliability and higher bulk at the same time.
The possible means of achieving higher bulk include the manufacturing of a bulky coated base paper through the use of a bulk pulp and bulk filler material, a reduction of the coat weight, and the lessening of surface treatment for the coated paper thus obtained.
Pulps for paper production are generally classified into chemical pulps and mechanical pulps. Chemical pulps are produced using a chemical that extracts the lignin from the fibers. Mechanical pulps, which are made without the use of chemicals, include the ground wood pulp—which is produced by grinding wood chips with a grinder—and the thermo-mechanical pulp, which is made by crumbling wood chips into fibers in a refiner. Generally, the mechanical pulp has stiffer fibers than the chemical pulp and is therefore more effective in providing higher bulk (lower density). However, the mechanical pulp will result in problems such as decreased whiteness if it's blended in a high-quality paper, and will easily cause printing defects such as picking due to shives if it's blended in a medium-quality paper. Thus there is a limit to the amount of mechanical pulp content that can be used in the paper. Furthermore, pulp from recycled paper is increasingly being used due to the recent public trend toward environmental preservation and the need to protect natural resources. Generally, however, recycled paper pulp is often produced by mixing fine paper, newsprint, magazine paper, coated papers and other used papers, and thus has a higher density than virgin mechanical pulp (unused pulp that has never made into paper) and cannot provide higher bulk.
As explained above, it is difficult to achieve sufficient paper bulk by working solely with pulp factors, especially when one considers the preservation of wood resources and the quality design of paper. Moreover, a simple blending of the above-described pulps for the sake of higher bulk results in greater stiffness, which makes it impossible to obtain sufficient pliability in the paper.
An example of the use of a bulky filler material in the base paper for use in a coated stock, in order to achieve higher bulk, is described in Japanese Patent Application Laid-open No. 5-339898, which discloses a technique used to achieve lower density through the blending of hollow synthetic organic capsules. However, such synthetic organic matter degrades the paper strength and causes printing problems such as picking and tearing, while a greater percentage of said matter needs to be blended to achieve a sufficient bulk effect, resulting in a higher production cost. A method of using a shirasu balloon is proposed in Japanese Patent Application Laid-open No. 52-74001. However, the shirasu balloon does not mix well with the pulp, and the paper blended with it causes print variations and other problems. In short, it is impossible to achieve pliability in the paper even through the use of any of the techniques so far discussed in this document.
The coating layer of the coated paper generally has a higher density than the base paper. Therefore, the coated paper has a higher density than the printing paper with no coating layer. A coated paper with higher bulk may be achieved by applying a smaller amount of coating composition. This is due to a smaller percentage of the coating layer relative to the overall coated paper. However, there has traditionally been a limit to the use of the coating layer in a smaller percentage as a means of reducing the amount of coating while maintaining the target quality, since it will also diminish the coverage of the base paper by the coating layer, thereby reducing the print quality such as white-paper gloss, smoothness and print gloss.
Enhancing the smoothness of the coated paper is one of the effective means of improving the print quality of the coated paper, particularly the degree of ink receptivity and gloss of the image area (hereinafter referred to as “print gloss”). Therefore, the process of smoothing the surface of the paper, such as super-calendering or soft nip-calendering, is generally used for glossy paper and the dull-coat paper having a level of white-paper gloss falling between those of the matte and glossy papers. However, such processes involve pressing the paper to achieve a smoother surface, thereby reducing the paper thickness and often making it impossible to gain a degree of bulk sufficient to achieve the target print quality.
The method of manufacturing general matte coated papers, on the other hand, is mainly intended to minimize sheet gloss, and therefore has conventionally used coatings blended with pigments having higher average particle diameters. For example, the primary pigments used in the coating disclosed in Japanese Patent Application Laid-open No. 8-60597 feature larger particle diameters and include 30 parts by weight of Escalon 1500, a type of ground calcium carbonate (average particle diameter: 1.65 μm) and 50 parts by weight of Hydrasperse, a No.2 kaoline (average particle diameter: 1.61 μm), thereby making it difficult to increase the smoothness, white-paper gloss and print gloss of the paper to the respective target levels.
The dull-coat paper, which is generally obtained through the application of a slight surface treatment to the matte coated paper, provides a higher print gloss than the matt coated paper but requires the enhancement of surface-treatment conditions if greater print gloss must be obtained. Therefore, as with the case of matte coated paper, it has been difficult to maintain the bulky feel of the dull-coat paper by manufacturing a stock of lower density. For example, as is disclosed in Japanese Patent Application Laid-open No. 7-119086, there is a technique for improving smoothness while minimizing white-paper gloss by selecting a higher roughness setting for the roller surface of the super-calender, which is commonly used as a surfacetreatment device. However, if the paper is finished with a calender having a stack of six or more rolls, the paper's density increases and bulk decreases, making it impossible to obtain a matte coated paper having the target bulk level.
Additionally, one technique for improving print gloss while producing a lower density and minimizing the sheet gloss is the use of a calender combining metal and resin rollers having rough surfaces. It is the process of surface treatment at a temperature of 100° C. using metal rollers having rough surfaces, as disclosed in, for example, Japanese Patent Application Laid-open Nos. 6-73685, 6-73686, 6-73697 and 7-238493. However, even with the use of such technologies it remains difficult to obtain a printing paper that offers the level of bulk targeted in the present invention.
Given the above circumstances, the purpose of the present invention is to provide a coated printing paper that provides higher bulk (lower density) yet excellent pliability, great workability with the printing machinery, higher print gloss regardless of lower sheet gloss, minimal small-scale gloss variations in the image area, and superior print quality.